In an activated sludge plant, what elements must be balanced to maintain steady-state concentrations?

• A. Electrical power, ventilation, and lighting
• B. Temperature, humidity, and pH
• C. Influent load, biological consumption, and sludge wasting.
• D. Turbidity, color, and odor

Answer: (C)

In an activated sludge plant, steady-state concentrations are achieved by balancing what comes into the system, what the microbes use, and what is removed as waste. The influent load sets the amount of substrate available for biological growth. Biological consumption represents the microbes breaking down organics and forming biomass, which dictates how quickly substrate and biomass are used up. Sludge wasting then removes excess biomass and solids from the system to keep the overall solids concentration and food-to-microorganism ratio under control. When these three rates are in balance, substrate and biomass levels remain steady, supporting stable treatment performance.

The other options describe operational or quality factors (power, ventilation, lighting; environmental conditions like temperature and pH; or effluent attributes like turbidity, color, and odor) that affect plant operation or output but do not define the fundamental balance needed for steady-state concentrations.